Apparatus and methods for calibrating optical measurements

Abstract

Apparatus and methods are described for calibrating an optical system that is used for measuring optical properties of a portion of a subjects body. During a calibration stage, a front surface of a calibration object ( 300 ) is illuminated, light reflected from a plurality of points on the calibration object ( 300 ) is detected, and intensities of the light reflected from the plurality of points on the calibration object ( 300 ) are measured. During a measurement stage, the portion of the subjects body is illuminated, and light reflected from the portion of the subjects body is detected. Measurements performed upon the light that was reflected from the portion of the subjects body are calibrated, using the measured intensities of the light reflected from the plurality of points on the calibration object ( 300 ). Other applications are also described.

The invention claimed is: 
     
       1. A method for calibration of an optical system used for measuring optical properties of a portion of a body of a subject, the method comprising:
 during a calibration stage, deriving information regarding characteristics of the optical system, with reference to which the measurements that are performed upon the portion of the subject's body may be calibrated, by:
 illuminating a front surface of a calibration object having known reflectance characteristics with light from a light source of the optical system, the front surface being at least partially curved; 
 detecting light reflected from a plurality of points on the calibration object, using at least one detector of the optical system; 
 measuring intensities of the light reflected from the plurality of points on the calibration object; and 
 determining a calibration value at each location upon the calibration object by calculating an integral over all angles with the known reflectance of the calibration object to determine the absolute reflection coefficient; and 
 
 during a measurement stage:
 illuminating the portion of the subject's body with light from the light source; 
 detecting light reflected from the portion of the subject's body, using the at least one detector; and 
 calibrating measurements performed upon the light reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object. 
 
 
     
     
       2. The method according to  claim 1 , wherein the calibration object further includes a back surface that is configured to have a lower reflectance that the front surface, such that a ratio of a reflectance of the front surface to the reflectance of the back surface is greater than 10:1. 
     
     
       3. The method according to  claim 1 ,
 further comprising, during the measurement stage:
 receiving an input indicating a shape of the portion of the subject's body; and 
 selecting one calibration object out of a plurality of differently-shaped calibration objects as best corresponding to the shape of the portion of the subject's body, 
 
 wherein measuring intensities of the light reflected from the plurality of points on the calibration object comprises measuring intensities of the light reflected from the plurality of points on the selected calibration object. 
 
     
     
       4. The method according to  claim 1 , further comprising during the measurement stage, determining a disposition of the portion of the subject's body relative to the optical system, wherein calibrating measurements performed upon the light that was reflected from the portion of the subject's body comprises calibrating measurements performed upon the light that was reflected from the portion of the subject's body, based upon the determined disposition of the portion of the subject's body relative to the optical system and the intensities of the light reflected from the plurality of points on the calibration object as measured during the calibration stage. 
     
     
       5. The method according to  claim 1 , wherein:
 measuring intensities of the light reflected from the plurality of points on the calibration object comprises measuring respective intensities of a plurality of wavelengths of light reflected from the plurality of points on the calibration object; 
 detecting light reflected from the portion of the subject's body using the at least one detector comprises detecting respective wavelengths of light reflected from the portion of the subject's body; and 
 calibrating measurements performed upon the light that was reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object comprises calibrating respective wavelengths of light that were reflected from the portion of the subject's body using the measured intensities of the respective corresponding wavelengths of light reflected from the plurality of points on the calibration object. 
 
     
     
       6. The method according to  claim 1 , wherein:
 measuring intensities of the light reflected from the plurality of points on the calibration object comprises measuring respective intensities of a plurality of wavelength-bands of light reflected from the plurality of points on the calibration object; 
 detecting light reflected from the portion of the subject's body using the at least one detector comprises detecting respective wavelength-bands of light reflected from the portion of the subject's body; and 
 calibrating measurements performed upon the light that was reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object comprises calibrating respective wavelength-bands of light that were reflected from the portion of the subject's body using the measured intensities of the respective corresponding wavelength-bands of light reflected from the plurality of points on the calibration object. 
 
     
     
       7. The method according to  claim 1 ,
 further comprising, during the calibration stage, moving the calibration object to a plurality of locations with respect to the optical system, 
 wherein:
 illuminating the front surface of the calibration object with light from the light source of the optical system comprises illuminating the front surface of the calibration object with light from the light source of the optical system, while the calibration object is at each of the plurality of locations; 
 detecting light reflected from the plurality of points on the calibration object, using the at least one detector of the optical system comprises detecting light reflected from the plurality of points on the calibration object, using the at least one detector of the optical system while the calibration object is at each of the plurality of locations; and 
 measuring intensities of the light reflected from the plurality of points on the calibration object comprises measuring intensities of the light reflected from the plurality of points on the calibration object, while the calibration object is at each of the plurality of locations. 
 
 
     
     
       8. The method according to  claim 7 , wherein moving the calibration object to the plurality of locations with respect to the optical system comprises moving the calibration object to a plurality of locations along an optical axis of the optical system. 
     
     
       9. The method according to  claim 7 , wherein moving the calibration object to the plurality of locations with respect to the optical system comprises moving the calibration object to a plurality of locations with respect to a plane that is perpendicular to an optical axis of the optical system. 
     
     
       10. The method according to  claim 7 , wherein moving the calibration object to the plurality of locations with respect to the optical system comprises moving the calibration object to a plurality of locations along an optical axis the optical system, and to a plurality of locations with respect to a plane that is perpendicular to the optical axis of the optical system. 
     
     
       11. The method according to  claim 7 , further comprising during the measurement stage, determining a disposition of the portion of the subject's body relative to the optical system, wherein calibrating measurements performed upon the light that was reflected from the portion of the subject's body comprises calibrating measurements performed upon the light that was reflected from the portion of the subject's body, based upon the determined disposition of the portion of the subject's body relative to the optical system and the intensities of the light reflected from the plurality of points on the calibration object that were measured during the calibration stage. 
     
     
       12. The method according to  claim 7 , further comprising:
 during the calibration stage acquiring one or more reference intensity measurements using a reference calibration object; and 
 subsequent to the calibration stage, performing periodic spectral-variation calibration measurements, in order to account for spectral variations in the light emitted by the light source over time, by:
 directing light from the light source toward the reference calibration object; 
 detecting light reflected from the reference calibration object, using the at least one detector; and 
 comparing spectral characteristics of the light reflected from the reference calibration object to the one or more reference intensity measurements. 
 
 
     
     
       13. The method according to  claim 12 , wherein performing periodic spectral-variation calibration measurements comprises performing periodic spectral-variation calibration measurements at a time interval of at least one every 30 minutes. 
     
     
       14. The method according to  claim 12 , wherein performing periodic spectral-variation calibration measurements comprises performing periodic spectral-variation calibration measurements each time the measurement stage is performed with respect to a new body portion of a subject. 
     
     
       15. The method according to  claim 12 , wherein performing periodic spectral-variation calibration measurements comprises performing periodic spectral-variation calibration measurements each time the measurement stage is performed with respect to a new subject. 
     
     
       16. The method according to  claim 12 , wherein the light source includes a broadband light source, and wherein performing the periodic spectral-variation calibration measurements, in order to account for spectral variations in the light emitted by the light source over time comprises performing periodic spectral-variation calibration measurements, in order to account for spectral variations in the light emitted by the broadband light source over time. 
     
     
       17. The method according to  claim 16 , wherein the light source includes a broadband light source that is configured to emit light within a range of 400 nm to 1,000 nm, and wherein performing the periodic spectral-variation calibration measurements, in order to account for spectral variations in the light emitted by the light source over time comprises performing periodic spectral-variation calibration measurements, in order to account for spectral variations in the light emitted by the broadband light source over time. 
     
     
       18. The method according to  claim 1 , wherein:
 the portion of the subject's body includes a cornea of an eye of the subject; 
 the curvature of the front surface of the calibration object corresponds to a curvature of the cornea; 
 illuminating the portion of the subject's body with light from the light source comprises illuminating the subject's cornea with light from the light source; 
 detecting light reflected from the portion of the subject's body using the at least one detector comprises detecting light reflected from the subject's cornea using the at least one detector; and 
 calibrating measurements performed upon the light reflected from the portion of the subject's body using the measured intensities of the light reflected from the plurality of points on the calibration object comprises calibrating measurements performed upon the light reflected from the subject's cornea using the measured intensities of the light reflected from the plurality of points on the calibration object. 
 
     
     
       19. Apparatus comprising:
 an optical system configured for measuring optical properties of a portion of a body of a subject, the optical system comprising a light source, and at least one detector; 
 a calibration object that defines a front surface that is at least partially curved, and that has known reflectance characteristics; and 
 at least one computer processor configured:
 during a calibration stage, to derive information regarding characteristics of the optical system, with reference to which the measurements that are performed upon the portion of the subject's body may be calibrated, by:
 detecting light from the light source that is reflected from a plurality of points on the calibration object, using the at least one detector, 
 measuring intensities of the light reflected from the plurality of points on the calibration object, and 
 determining a calibration value at each location upon the calibration object by calculating an integral over all angles with the known reflectance of the calibration object to determine the absolute reflection coefficient, and during a measurement stage: 
 to detect light from the light source that is reflected from the portion of the subject's body, using the at least one detector, and 
 to calibrate measurements of the light that was reflected from the portion of the subject's body, using the measured intensities of the light reflected from the plurality of points on the calibration object.